New publication: Exercise training increases skeletal muscle mitochondrial
volume density by enlargement of existing mitochondria
and not de novo biogenesis

A.-K. Meinild Lundby, R. A. Jacobs, S. Gehrig, J. de Leur, M. Hauser, T. C. Bonne,
D. Flück, S. Dandanell, N. Kirk, A. Kaech, U. Ziegler, S. Larsen and C. Lundby

Aims: (i) To determine whether exercise-induced increases in muscle mitochondrial
volume density (MitoVD) are related to enlargement of existing
mitochondria or de novo biogenesis and (ii) to establish whether measures
of mitochondrial-specific enzymatic activities are valid biomarkers for
exercise-induced increases in MitoVD.
Method: Skeletal muscle samples were collected from 21 healthy males
prior to and following 6 weeks of endurance training. Transmission electron
microscopy was used for the estimation of mitochondrial densities
and profiles. Biochemical assays, western blotting and high-resolution
respirometry were applied to detect changes in specific mitochondrial functions.
Result: MitoVD increased with 55  9% (P < 0.001), whereas the number
of mitochondrial profiles per area of skeletal muscle remained unchanged
following training. Citrate synthase activity (CS) increased (44  12%,
P < 0.001); however, there were no functional changes in oxidative phosphorylation
capacity (OXPHOS, CI+IIP) or cytochrome c oxidase (COX)
activity. Correlations were found between MitoVD and CS (P = 0.01;
r = 0.58), OXPHOS, CI+CIIP (P = 0.01; R = 0.58) and COX (P = 0.02;
R = 0.52) before training; after training, a correlation was found between
MitoVD and CS activity only (P = 0.04; R = 0.49). Intrinsic respiratory
capacities decreased (P < 0.05) with training when respiration was normalized
to MitoVD. This was not the case when normalized to CS activity
although the percentage change was comparable.
Conclusions: MitoVD was increased by inducing mitochondrial enlargement
rather than de novo biogenesis. CS activity may be appropriate to
track training-induced changes in MitoVD.
Keywords adaptations, mitochondria, muscle, training, volume density.